Method for warming up a catalytic converter arranged downstream from a spark-ignition, direct injection internal combustion engine

ABSTRACT

The invention relates to a method for warming up at least one catalytic converter that is arranged downstream from a spark-ignition, direct injection internal combustion engine. To this end, an exhaust gas temperature is at least temporarily increased by at least one measure, which is executed by the engine, after the conclusion (t 1 ) of the engine start of the internal combustion engine. The measures, which are executed by the engine, consist of a multiple injection during which at least two fuel injections into the cylinder are carried out within an induction cycle and compression cycle of a cylinder of the internal combustion engine, and/or said measures consist of a spark retarding. The invention provides that the measure executed by the engine or a combination of measures having the strongest heating action is available, at the earliest, after a retardation of at least two working cycles of the internal combustion engine after the conclusion (t 1 ) of the engine start. The method makes it possible to keep jumps in torque low, which result from the initiation of heating measures, and to provide these in a manner that can be reproduced and easily regulated. Moreover, a reliable ignition and combustion can be guaranteed during the entire warming up phase.

[0001] The invention relates to a method for warming up at least onecatalytic converter arranged downstream of a spark-ignition, directinjection internal combustion engine, in particular after an enginestart of the internal combustion engine, with the features of thepreambles of the independent claims.

[0002] Catalytic converters are used in exhaust gas ducts of internalcombustion engines for converting pollutants contained in the exhaustgases of the internal combustion engine into substances that are lessharmful to the environment. The catalytic converters must be warmed upto at least a start or light-off temperature specific for catalyticconverter in order to maintain their service readiness. The term starttemperature hereby refers to a temperature where the catalytic converterhas a conversion efficiency of 50%. Until the time after an engine coldstart when the catalytic converter reaches its start temperature,pollutants in the exhaust gas can enter the atmosphere essentiallyunconverted. Several strategies are known for increasing an exhausttemperature and thereby accelerating catalytic converter warm-up.

[0003] It is known to retard an ignition angle, i.e., the time when anair-fuel mixture in a cylinder is ignited, relative to an ignition anglethat provides the highest efficiency during the warm-up phase.Retardation of the ignition angle reduces the efficiency of thecombustion while simultaneously increasing an exhaust gas temperature.The hotter exhaust gas causes the catalytic converters to heat upfaster. The method of retarding ignition reaches its limits at ignitionangles where the internal combustion engine begins to run unacceptablyrough and reliable ignition can no longer be guaranteed.

[0004] Another method for increasing the exhaust gas temperatureincludes so-called multiple injection which has recently been describedfor direct-injection, spark-ignition internal combustion engines, wherethe fuel is injected directly through injection valves into a combustionchamber of a cylinder (WO 00/08328, EP 0 982 489 A2, WO 00/57045). Inthis case, a total fuel quantity to be supplied during an operatingcycle of a cylinder is divided into two parts and supplied in twoinjection processes to the combustion chamber of the cylinder. A firstearly injection (homogeneous injection) takes place during an intakestroke of the cylinder so that the injected fuel quantity is at thefollowing ignition time at least substantially homogeneously distributedin the combustion chamber. On the other hand, a second late injection(stratified injection) is carried out during a following compressionstroke, in particular during the second half of the compression stroke,resulting in a so-called stratified charge where the injected fuel cloudis essentially concentrated in the region surrounding a spark plug ofthe cylinder. Accordingly, multiple injection operation of the internalcombustion engine involves a mixed operation of stratified charging andhomogeneous charging. The particular ignition characteristic of themultiple injection operation results in an increased exhaust gastemperature compared to a completely homogeneous operation. In additionto increasing the exhaust gas temperature, multiple injectionadvantageously also reduces raw emission of nitric oxides NO_(x) andunburned hydrocarbons HC, thereby reducing pollutant breakthrough duringthe warm-up phase.

[0005] Initiating and terminating the multiple injection, in other wordsthe transition from single to multiple injection operation and back,poses problems. In particular, the late stratified injection causes thefuel to be partial knocked off the piston head, the cylinder walls andthe spark plug due to the low-temperature of the internal combustionengine after engine start. The fuel that does not evaporate at the coldengine temperature is not available for the subsequent combustionprocess. As a result, misfirings and rough combustion processes occur.Another problem is the early ignition angle at the time of thechangeover to multiple injection which lies in particular before theupper dead center U.D.C. Since an injection end of the late injection isalso located in or shortly before this range, the mixture cannot beoptimally conditioned since there is not enough time available fortransporting the stratified cloud from the injector to the spark plug.As a result, higher HC raw emissions are observed.

[0006] It is an object of the present invention to develop a method forwarming up a catalytic converter whereby engine-related heatingmeasures, in particular a changeover into a multiple injection operationand back, occurs with the lowest possible emission of pollutants andminimal torque variations.

[0007] The object is solved by a method with the characterizing featuresof the independent claims 1, 3 and 4.

[0008] According to the invention, the engine-related measure orcombination of measures implemented during a warm-up with the mostprofound heating effect occurs at the earliest after a delay of at leasttwo operating cycles of the internal combustion engine, in particular ofat least three, preferably at least five operating cycles, after theconclusion of the engine start. The term conclusion of the engine startindicates here in the point in time where the engine speed, after arotation speed overshoot during the starting process, lies for the firsttime in a rotation speed range of 95 to 105% of a nominal idle rotationspeed. If no pronounced rotation speed overshoot occurs during thestartup phase due to the employed principle, then the conclusion of theengine start is interpreted as the point in time where the enginerotation speed is for the first time continuously for at least 0.5seconds in the rotation speed range of 95 to 105% of the nominal idlerotation speed. Moreover, the term operating cycle is to be understoodas performing once all operational processes of an internal combustionengine on one cylinder. In a four-cycle engine, these represent twocrankshafts revolutions.

[0009] Since an increasing heating effect, i.e., an increasing exhaustgas temperature, is necessarily associated with a decrease in the engineefficiency and therefore with an increasing torque loss, the method ofthe invention makes it possible to keep torque discontinuities small by,for example, switching and/or enhancing the heating measuressuccessively in the order of their potential heating effect. Even if theheating measure or measures are introduced in a single stage after adelay, the initial operation still causes the internal combustion engineto warm up and stabilize, so that the subsequent torque discontinuitybecomes more reproducible and more easily controllable, therebymaintaining a reliable ignition and combustion.

[0010] In particular, a single injection operation is performed during afirst phase of the warm-up after the conclusion of the engine start,wherein the ignition angle is at least temporary retarded, and achangeover to a multiple injection operation occurs during a subsequentsecond phase. The term retardation of the ignition angle is to beunderstood as each ignition point that occurs after an ignition pointwith the highest engine efficiency, which in particular results in areduced engine efficiency by at least 5%. The cylinder is thereforealready warmed up due to the ignition angle retardation during the firstphase, thereby effectively reducing so-called wall film problems thatoccur as a result of fuel condensation from the late injection inmultiple injection operation.

[0011] According to another modified embodiment of the method, anoperation with 30 to 100% of an injected fuel quantity occurs after theconclusion of the engine start with an essentially homogeneous mixturepreparation at an ignition time during a first phase, and a multipleinjection operation occurs in a subsequent second phase where at theignition time at least 35% of an injected fuel quantity are present as astratified charge and at least 20% of the fuel quantity in a homogeneousdistribution. Preferably, the first phase occurs during a completelyhomogeneous operation, whereby the total injected fuel quantity isavailable at the ignition time in form of an essentially homogeneousmixture. In the context of this invention, homogeneous operation is tobe understood as representing a fuel density distribution in thecombustion chamber at the time of ignition, where the highest fueldensity at a point in the combustion chamber deviates from the lowestfuel density at another location in the combustion chamber by less than30%. Such homogeneity can be achieved in a known manner by an injectionduring an intake stroke of the cylinder, in particular within the firsthalf of the intake stroke.

[0012] The multiple injection preferably includes two injections,whereby a first, early injection occurs essentially during an intakestroke of a cylinder, preferably during a first half of the intakestroke, and a second, late injection occurs during a subsequentcompression stroke, preferably during a second half of the compressionstroke. In this way, an essentially homogeneous distribution of the fuelsupplied in the early injection at the ignition time is achieved as wellas a stratified charge of the fuel supplied in the late injection whichis concentrated essentially in a region around a spark plug of thecylinder. Accordingly, the early injection will be referred to ashomogeneous injection and the late injection as stratified injection. Asalready described above, such mixed fuel processing simultaneouslyincreases the combustion and/or exhaust gas temperature and reduces araw emission of unburnt hydrocarbons and nitric oxides. The fuelfractions of the two injections are preferably selected so that thehomogeneous injection results in a very lean fuel mixture that cannot beignited by itself and can only be combusted with the help of thestratified charge of the second injection. To guarantee a completedcombustion of the homogeneous charge, the fuel quantity supplied duringhomogeneous injection should be no less than 20% of the total suppliedfuel quantity. Preferably, a slightly lean to stoichiometric air-fuelmixture with a lambda value between 1 and 1.2 is set during warm-up. Thelambda value during the multiple injection phase can be more stronglyshifted towards lean than during the preceding first phase of thewarm-up, taking advantage of the fact that the catalytic converter has alower startup temperature in a lean exhaust gas atmosphere than in astoichiometric atmosphere.

[0013] According to a particular advantageous embodiment of theinvention, the first phase is initially started with an early ignitionangle, in particular with an ignition angle before the upper dead center(U.D.C.), which preferably corresponds to the ignition angle selectedduring the engine start. This early ignition angle is thereafterprogressively retarded, in particular past the U.D.C. The progressiveretardation of the ignition angle can be continuous and/or stepwise.Preferably, when changing over to a multiple injection operation bytaking into consideration a torque compensation at the switching point,the approximate last ignition angle of the first phase is adopted andcontinuously and progressively retarded further. Preferably, thechangeover occurs at an ignition angle where a completely homogeneousoperation is still possible. In particular, the changeover should occuraround 6°, in particular around 4°, preferably around 2° before thiscritical ignition angle so as to permit corrections in the ignitionangle to compensate the torque change that occurs during the changeoverinto the multiple injection operation. The changeover to multipleinjection operation can occur preferably at an ignition angle of between0 and 20° after U.D.C., in particular of 10° after U.D.C., taking intoaccount the torque reserve. After changeover to the multiple injectionoperation, the ignition point should be progressively shifted towardsthe latest possible ignition angle that depends on the engine design. Amaximum ignition angle of 20 to 45° after U.D.C., in particular of 35°after U.D.C., should not be exceeded.

[0014] According to another improvement of the method, an injectionangle, or injection time, of the late stratified injection is alsoprogressively retarded after changeover into the multiple injectionoperation, which can take place essentially synchronously with theprogressive retardation of the ignition angle. Preferably, an injectionend of the late injection is shifted with an essentially constantdifference to the ignition angle of 50 to 100°, in particular of 60 to80°. This difference can also be varied as a function of the enginerotation speed and/or the injection pressure, which ensures a consistentoptimized time for mixture preparation of the stratified injection andtransport of the fuel cloud to the spark plug.

[0015] Advantageously, the multiple injection, for example after atleast partial warm-up of at least a first catalytic converter, can beterminated depending on an actual operating point of the internalcombustion engine. If the internal combustion engine is at that time atthe beginning of a load demand phase, for example in a startup and/or oracceleration phase, then the torque reserve expended for the heatingmeasure can be utilized immediately; the multiple injection and/or theretardation of the injection angle can be immediately terminated, andthe internal combustion engine can be switched over to a homogeneous orstratified operation. Conversely, if the internal combustion engine isin a constant load phase, for example in idle, after completing warm-up,then the heating measures are preferably reduced in the reverse order oftheir initiation. In particular, the injection angle of the stratifiedinjection and/or the ignition angle are progressively advanced, and achangeover into the single injection operation takes place as soon asthe ignition angle permits a homogeneous operation.

[0016] The time diagrams of all the aforedescribed methods, inparticular the initiation of those engine-related measures orcombination of measures with the most profound heating effect, thechangeover to multiple injection operation and/or or the identificationof the completed warm-up and reduction of the measures, can occur basedon a measured and/or or modeled engine and/or or exhaust gas and/orcatalytic converter temperature, and/or based on a time elapsedafter-the conclusion of the engine start and/or completed crankshaftrevolutions and/or a distance traveled and/or a cumulative exhaust gasheat flow.

[0017] Additional advantageous embodiment of the invention are recitedin the dependent claims.

[0018] Embodiments of the invention will be described hereinafter inmore detail with reference to the appended drawings. It is shown in:

[0019]FIG. 1 a time diagram of engine rotation speed, ignition angle andinjection angle during a warm-up phase according to a conventionalmethod;

[0020]FIG. 2 a time diagram of raw emission of unburnt hydrocarbonsduring a warm-up phase according to two methods that do not correspondto the invention;

[0021]FIG. 3 a time diagram of engine rotation speed, ignition angle andinjection angle according to a method for warming up a catalyticconverter according to a first embodiment of the invention;

[0022]FIG. 4 a time diagram of engine rotation speed, ignition angle andinjection angle according to a second embodiment of the invention;

[0023]FIG. 5 a time diagram of engine rotation speed, ignition angle andinjection angle according to a third embodiment of the invention;

[0024]FIG. 6 a time diagram of engine rotation speed, ignition angle andinjection angle during a termination of the warm-up according to afourth embodiment of the invention; and

[0025]FIG. 7 a time diagram of engine rotation speed, ignition angle andinjection angle during a termination of the warm-up according to a fifthembodiment of the invention.

[0026]FIG. 1 shows a method for warming up a catalytic converter locateddownstream of conventional a direct-injection internal combustionengine. The starter motor is energized before a time t₀, during whichtime the starter motor starts up, couples to the internal combustionengine and drives the engine initially to a minimum rotation speed. Anengine start phase takes place between t₀ and t₁, during which theengine rotation speed n of the internal combustion engine increases andthereafter settles in a range of a substantially constant nominal idlerotation speed. The internal combustion engine operates in homogeneousoperation until the begin of the engine start t₀, wherein a total fuelquantity to be supplied is injected in a single injection process duringan intake stroke of a cylinder (single injection). The ignition angleα_(Z) is hereby set to a crankshaft angle KWW before the upper deadcenter U.D.C., in particular to an ignition angle that ensures thehighest engine efficiency and/or or the highest starting reliability.

[0027] According to known methods, the internal combustion enginechanges over to a multiple injection operation already at the beginningof the engine run-up at the time t₀ in order to increase an exhaust gastemperature and accelerate the warm-up of the downstream catalyticconverter. A fraction of the fuel quantity is hereby injected during theintake stroke and hence is present as a homogeneous mixture at theignition time (homogeneous injection). The remaining fuel quantity isinjected in a second, late injection during a compression stroke, inparticular during the second half of the compression stroke (stratifiedinjection). According to the illustrated method, the injection angleα_(EE) for the late injection (stratified injection) is kept constantduring the warm-up. When the multiple injection begins, the ignitionangle α_(Z) is typically retarded, typically into the range around theupper ignition dead center U.D.C.

[0028] The aforedescribed approach provides, on one hand, only a shorttime interval between the injection time of the late stratifiedinjection and the ignition time and, on the other hand, a small distancebetween the injection valve and the piston head at the time ofinjection. As a result, the late injected fuel cannot be optimallyformed as a stratified cloud and transported into the region of thespark plug before the ignition time. Instead, the fuel is concentratedat the ignition time mainly in the region of the piston head in the formof a stratified cloud. Moreover, mixture preparation is adverselyaffected by the late injection in the multiple injection operation whichbegins immediately at the beginning of the engine start t₀, because theengine, in particular the piston head, is still cold and causes the lateinjected fuel to condensate on the piston head, the cylinder walls andthe spark plug, by preventing complete evaporation due to the coldtemperatures. As a result of the insufficiently short conditioning timefor the mixture and the cold combustion chamber, the internal combustionengine runs increasingly rough, shows noticeably higher HC raw emission,misfires and can even shut down. The heating measures are conventionallyterminated at the time t_(E) when they catalytic converter has reachedits light-off temperature. A noticeable torque discontinuity is observedat this time, as described above in conjunction with the initiation ofthe multiple injection operation.

[0029]FIG. 2 depicts measured curves of the raw emission of unburnthydrocarbons HC when the internal combustion engine is operatingaccording to two methods for warming up the catalytic converter that arenot part of the invention. The measurements are performed after anengine cold start at 20° C. and with the depicted vehicle speed profilev_(FZG) conforming to the “New European Driving Cycle” NEFZ. Thestarting point of the time axis corresponds to the conclusion of themotor (time t₁ in FIG. 1), i.e., the time where the engine rotationspeed, after a rotation speed overshoot, lies during the start processfor the first time in the range of a nominal idle rotation speed ±5%.The air-fuel ratio was in both cases controlled to a slightly leanlambda value between 1.0 and 1.1. The curve HC_(SZ) shows the HC rawemission of the internal combustion engine with conventional retardedignition with a latest ignition angle of approximately 10° after U.D.C.in single injection operation with homogeneous mixture conditioning. Thecurve HC_(ME), on the other hand, shows the HC raw emission in multipleinjection operation with 50% of the injected fuel quantity inhomogeneous and 50% in stratified fuel conditioning. The control end ofthe late injection was about 40° before U.D.C. with a latest ignitionangle of 27° after U.D.C. In both measurements, the respective heatingmeasures, i.e. late ignition (HC_(SZ)) and multiple injection/lateignition (HC_(ME)), where initiated immediately after the conclusion ofthe motor start t₁. Most curves show profound HC emission maxima atapproximately 3 to 4 seconds. This indicates that a multiple injectionoperation causes a much higher observable pollutant emission that aconventional retarded ignition. When the heating measures are initiated,the ignition angle is still relatively early at approximately 10° beforeU.D.C. During a time interval of approximately 1.5 to 3 seconds theignition angle is shifted to the latest ignition angle commensurate withthe heating measures. In conventional late ignition, the essentiallyhomogeneous mixture preparation provides a sufficiently high ignitionreliability over the entire ignition angle window between 20° beforeU.D.C.. and 10° after U.D.C., so that a maximum HC emission of onlyapproximately 100 g/h is reached. Conversely, if the heating measureincludes multiple injection, then no ignition-safe mixture preparationis possible at the beginning of the heating measure, i.e., at anignition angle of 10° before U.D.C. The mixture preparation is improvedonly during the shift of the ignition angle to the latest possibleignition angle, with the HC emissions decreasing due to the higherlikelihood of ignition. The maximum HC emission is with 190 g/hsignificantly higher and also extends over a longer time than HCemission with conventional late ignition. As also indicated, beneficialmixture preparation is not possible with conventional ignition angles ina range around the U.D.C. (see FIG. 1).

[0030]FIG. 3 shows the initiation of the engine heating measuresaccording to a first embodiment of the invention. Shortly after theconclusion of the engine start (t₁), a heat demand for a catalyticconverter is identified at a time t₂, for example based on a measured ormodeled catalytic converter temperature. The multiple injectionoperation is then initiated with an early homogeneous injection duringthe intake stroke and a late stratified injection during the compressionstroke, whereby the control end of the stratified injection α_(EE) isinitially set to a very early time, for example to 60 to 80° beforeU.D.C. The control end α_(EE) of the stratified injection is thereaftercontinuously retarded. When the multiple injection operation begins attime t₂ the ignition angle α_(Z), which was located at the conclusion ofthe engine start, for example, at 10°. before U.D.C., is progressivelyretarded. The injection angle α_(EE) and the ignition angle α_(Z) areessentially retarded synchronously, preferably with a constantseparation relative to each other of the 50 to 100°, in particular 60 to80°, wherein this separation can be varied depending on the enginerotation speed n and/or and injection pressure. This provides sufficienttime for conditioning the mixture. At a time t₄ the injection angleα_(EE) and the ignition angle α_(Z) have reached their allowed maximumvalues for a particular heating power. These settings are, for example,for the control end of the injection angle α_(EE) at 40° before U.D.C.and for the ignition angle α_(Z) of 20 to 30° after U.D.C. The heatingmeasures have therefore the most profound heating effect after the timet₄. The progressive increase of the heating effect between the times t₂and t₄ make it possible to obtain a stronger maximum heating effectcompared to the state-of-the-art. The slowly decreasing efficiency,starting at the time t₂, effectively prevents strong torque variations.The initially very early injection angle α_(EE) also counteractsprecipitation on the piston head of the fuel injected during the secondinjection. The piston head is at this time located far away from theinjection valve.

[0031] The formation of the stratified charge of the late injection inmultiple injection operation as well as its transport to the spark plugcan be promoted by suitably shaping the surface of the piston head, inparticular by forming troughs in the piston head, as well as bygenerating suitable air current conditions in the combustion chamber.These measures are known from stratified direct-injection internalcombustion engines and will therefore not be further explained. Theembodiment of the method depicted in FIG. 3 is particularly advantageousfor internal combustion engine which generate the stratified operationpredominantly through a tumble gas flow. This method is only sub-optimalfor direct injection engines that operate with a mixture conditioningprocess with a high swirl flow ratio, since with the piston located nearthe bottom, the injection cloud in the stratified injection is notperfectly diverted towards the spark plug and unravels at the time ofignition, preventing an ideal combustion process and increasing HCemissions.

[0032] An advantageous improvement of the method is depicted in FIG. 4.In this embodiment, the ignition angle α_(Z) is first progressivelyretarded during a first phase at the time t₂ when the heat demand of thecatalytic converter is identified. This represents a first heatingmeasure. The internal combustion engine is hereby operated with a mainfuel fraction, preferable the entire fuel, homogeneously distributed inthe combustion chamber, whereby the fuel is preferably injected duringthe intake stroke. The multiple injection operation is initiated at atime t₃ at an ignition angle of 0° to 20° after U.D.C., preferably at10° after U.D.C. This range of ignition angles also represents a limitup to which a stable homogeneous operation is still possible.Particularly advantageously, changeover into the multiple injectionoperation occurs several degrees before the latest possible homogeneousignition angle in order to retain a torque reserve for an eventuallyrequired adjustment of the ignition angle after a changeover to multipleinjection. The ignition angle α_(Z) during the single injectionoperation is adopted for multiple injection operation, taking intoconsideration the above ignition angle adjustment. The ignition angleα_(Z) is also progressively retarded after the changeover at time t₃,until a latest possible injection angle α_(Z) is reached at the time t₄.This injection angle is at most approximately 35° after U.D.C. Certaintorque reserves should also be considered (approximately ±2°) foradjusting the torque, for example, in idle. The control end α_(EE) ofthe stratified injection is in this case set from the start to a desiredsetting of, for example, 40° before U.D.C. and then kept unchanged,since the separation between the ignition angle α_(Z) and the injectionangle α_(EE) is adequate from the beginning (t₃) of the multipleinjection. This ensures a sufficient mixture preparation and optimalcombustion, also during the multiple injection operation. A maximumexhaust gas temperature in this example is reached at the time t₄ with adelay of several operating cycles after the conclusion of the enginestart at time t₁.

[0033] Another advantageous improvement of the method is depicted inFIG. 5. The ignition angle α_(Z) is hereby controlled essentially in thesame manner as in the preceding example. The multiple injectionoperation is again initiated at an ignition angle α_(Z) of approximately10° after U.D.C. A control end of the injection angle α_(EE) for thestratified injection is preferably initially set at 50 to 70° beforeU.D.C.. The ignition angle α_(EE) is then progressively retarded insynchronism with the injection angle α_(Z), until the injection angleα_(EE) and the ignition angle α_(Z) have reached their desired ranges atthe time t₄, providing maximum heating power. From this time on, bothangles are held at a constant value. During the entire multipleinjection operation, the injection end α_(EE) and the ignition angleα_(Z) have an essentially constant separation of 50 to 100°, preferably60 to 80°, optionally depending on the engine rotation speed and/or theinjection pressure, so that an optimal mixture preparation is achieved.The exemplary embodiment depicted FIG. 5 represents an optimizedsolution with respect to torque neutrality, pollutant emission andsmoothness of running.

[0034] The respective engine-related heating measures are preferablydecreased in the reverse order in which they were initiated, as long asthe internal combustion engine operates mostly with constant loaddemand, preferably in idle. A corresponding embodiment is shown in FIG.6. At the time t₅, a sufficient warm-up of the catalytic converter,preferably a pre-catalytic converter, is identified based on a measuredand/or modeled catalytic converter temperature. Alternatively, thispoint can also be identified based on an elapsed time after theconclusion of the engine start t₁, a number of revolutions since theconclusion of the engine start, a traveled distance and/or andintroduced heat flow. At the time t₅, both the control end α_(EE) of thestratified injection and the ignition angle α_(Z) are steadily advancedwith an essentially constant separation that can depend on the operatingpoint. As soon as an ignition angle α_(Z) is reached that permits, forexample, a homogeneous operation, the multiple injection is terminated(time t₆). The ignition angle α_(Z) is thereafter progressively fartheradvanced until a desired ignition angle that depends on the actualoperating point of the internal combustion engine is approximatelyreached.

[0035] Conversely, if the end of the warm-up is reached at a time when apositive load demand has built-up, for example at the beginning of astart up or an acceleration phase, then all heating measures can beimmediately decreased at time t_(E), as shown in FIG. 7, and theavailable charge can be used directly for changing the load demand. Moreparticularly, the multiple injection operation is terminated and theignition angle α_(Z) is set to the optimal range for the actualoperating point.

LIST OF REFERENCE NUMERALS

[0036] α_(Z) ignition angle

[0037] α_(EE) controlling stratified injection (injection angle)

[0038] HC_(ME) HC raw emission with catalytic converter warm-upaccording to conventional multiple injection

[0039] HC_(SZ) HC raw emission with catalytic converter warm-upaccording to conventional retarded ignition

[0040] KWW crankshaft angle

[0041] n engine rotation speed

[0042] t time

[0043] t₁ engine start end

[0044] v_(FZG) vehicle speed

[0045] U.D.C. upper dead center

1. Method for warming up at least one catalytic converter arrangeddownstream of a spark-ignition, direct injection internal combustionengine, whereby after a conclusion of an engine start (t₁) of theinternal combustion engine an exhaust gas temperature is increased atleast temporarily by at least one engine-related measure, and wherebythe engine-related measures comprise a multiple injection, wherein atleast two fuel injections into the cylinder are performed within anintake and compression stroke of a cylinder of the internal combustionengine and/or a retardation of an ignition angle, characterized in thatthe engine-related measure or combination of measures with the mostprofound heating effect occurs at the earliest after a delay of at leasttwo operating cycles of the internal combustion engine after theconclusion of the engine start (t₁).
 2. Method according to claim 1,characterized in that the engine-related measure or the combination ofmeasures with the most profound heating effect occurs at the earliestafter a delay of at least three, in particular of at least five,operating cycles of the internal combustion engine after the conclusionof the engine start (t₁).
 3. Method for warming up at least onecatalytic converter arranged downstream of a spark-ignition, directinjection internal combustion engine, whereby after an end of an enginestart (t₁) of the internal combustion engine an exhaust gas temperatureis increased at least temporarily by at least one engine-relatedmeasure, and the engine-related measures comprise a multiple injection,wherein at least two fuel injections into the cylinder are performedwithin an intake and compression stroke of a cylinder of the internalcombustion engine, and/or a retardation of an ignition angle,characterized in that after the conclusion of the engine start (t₁), asingle injection operation with at least temporary retardation of theignition angle is performed during a first phase, and a changeover intoa multiple injection operation occurs during a subsequent second phase.4. Method for warming up at least one catalytic converter arrangeddownstream of a spark-ignition, direct injection internal combustionengine, whereby after an end of an engine start (t₁) of the internalcombustion engine an exhaust gas temperature is increased at leasttemporarily by at least one engine-related measure, and theengine-related measures comprise a multiple injection, wherein at leasttwo fuel injections into the cylinder are performed within an intake andcompression stroke of a cylinder of the internal combustion engine,and/or a retardation of an ignition angle, characterized in that afterthe conclusion of the engine start (t₁), an operation with 30 to 100% ofan injected fuel quantity occurs during a first phase with anessentially homogeneous mixture preparation at an ignition time, and amultiple injection operation occurs in a subsequent second phase,wherein at the ignition time at least 35% of an injected fuel quantityis present as stratified charge and at least 20% of the fuel quantity ispresent in homogeneous distribution.
 5. Method according to one of thepreceding claims, characterized in that the multiple injection comprisestwo injections, wherein a first early injection occurs essentiallyduring an intake stroke and a second, late injection occurs during asubsequent compression stroke, and that the fuel supplied during theearly injection has at the ignition time an essentially homogeneousdistribution in the combustion chamber of the cylinder and the fuelsupplied during the late injection is concentrated at the ignition timeessentially as a stratified charge in a region around a spark plug. 6.Method according to claim 5, characterized in that the early injectiontakes place in particular during a first half of the intake stroke andthe late injection during a second half of the compression stroke. 7.Method according to one of the claims 3 to 6, characterized in that thefirst phase is started initially with an early ignition angle (α_(Z)),in particular with an ignition angle (α_(Z)) before the upper deadcenter (U.D.C.), and that thereafter the ignition angle is adjustedprogressively, continuously and/or stepwise towards a later ignitionpoint, in particular up to an ignition angle (α_(Z)) after the upperdead center (U.D.C.).
 8. Method according to claim 7, characterized inthat the changeover to a multiple injection operation occurs at anignition angle (α_(Z)) of between 0 and 20° after U.D.C., in particularof 10° after U.D.C.
 9. Method according to claim 8, characterized inthat the changeover into a multiple injection operation occurs around4°, in particular around 2° before an ignition angle (α_(Z)) at which anexclusively homogeneous operation is still barely possible.
 10. Methodaccording to one of the claims 7 to 9, characterized in that at thechangeover to the multiple injection operation the last ignition angle(α_(Z)) of the first phase is adopted and the progressive retardation ofthe injection angle is continued after the changeover to the multipleinjection operation.
 11. Method according to claim 10, characterized inthat the progressive retardation of the injection angle occurs at amaximum ignition angle (α_(Z)) of 20 to 45° after U.D.C., in particularat a maximum ignition angle (α_(Z)) of 35° after U.D.C.
 12. Methodaccording to one of the preceding claims, characterized in that aninjection angle (α_(EE)) of the late injection of the multiple injectionis retarded essentially synchronously with the progressive retardationof the ignition angle.
 13. Method according to claim 12, characterizedin that an injection end (α_(EE)) of the late injection is adjusted withan essentially constant difference of 50 to 100°, in particular of 60 to80°, relative to the ignition angle (α_(Z)).
 14. Method according toclaim 13, characterized in that the difference between the injection end(α_(EE)) of the late injection and the injection angle (α_(Z)) is variedas a function of the engine rotation speed (n) and/or an injectionpressure.
 15. Method according to one of the claims 12 to 14,characterized in that the injection end of the late injection isinitially set to an angle (α_(EE)) of 40 to 90° before U.D.C., inparticular 50 to 80° before U.D.C., and subsequently adjusted to anangle (α_(EE)) of 30 to 50° before U.D.C., in particular 40° beforeU.D.C.
 16. Method according to one of the preceding claims,characterized in that the initiation of the engine-related measure orthe combination of measures with the most profound heating effect and/orthe changeover to the multiple injection operation is identified basedon a measured and/or or modeled engine and/or exhaust gas and/orcatalytic converter temperature, and/or a time elapsed after theconclusion of the engine start and/or since a number of crankshaftrevolutions after the conclusion of the engine start and/or a distancetraveled after the conclusion of the engine start and/or a cumulativeexhaust gas heat flow since the conclusion of the engine start. 17.Method according to one of the preceding claims, characterized in thatwhen the internal combustion engine is in a phase with an essentiallyconstant load, in particular in idle, after at least partial warm-up ofat least a first catalytic converter, the ignition angle (α_(Z)) and/orthe injection angle (α_(EE)) is progressively advanced and the multipleinjection is terminated and a changeover to the single injectionoperation occurs as soon as the ignition angle (α_(EE)) permits ahomogeneous operation.
 18. Method according to claim 17, characterizedin that the changeover to the single injection operation occurs at anignition angle (α_(Z)) of 5 to 15° after U.D.C., in particular 10° afterU.D.C.
 19. Method according to claim 17 or 18, characterized in that theprogressive advancement of the ignition angle continues after thechangeover to the single injection operation.
 20. Method according toone of the preceding claims, characterized in that when the internalcombustion engine is in a load demand phase, in particular in a startupand/or acceleration phase, after at least partial warm-up of at least afirst catalytic converter, the multiple injection and/or the retardationof the ignition angle is immediately terminated and a changeover to asingle injection operation occurs.
 21. Method according to one of theclaims 17 to 20, characterized in that the completed warm-up isidentified based on a measured and/or or modeled exhaust gas and/orcatalytic converter temperature and/or a time elapsed after theconclusion of the engine start and/or since a number of crankshaftrevolutions after the conclusion of the engine start and/or a distancetraveled after the conclusion of the engine start and/or a cumulativeexhaust gas heat flow since the conclusion of the engine start.